Gear pumps as known in the art are particularly advantageous for pumping fluids while keeping the fluids isolated from the external environment. This benefit has been further enhanced by the advent of magnetically coupled drive mechanisms which have eliminated leak-prone hydraulic seals around drive shafts.
Gear pumps have been adapted for use in many applications, including applications requiring extremely accurate delivery of a liquid to a point of use. Such applications include, for example, delivery of liquids in medical instrumentation and delivery of liquid ink to continuous ink-jet printer heads.
Continuous ink-jet printing is rapidly becoming the method of choice for on-line application of text, such as on alphanumeric production code or bar code, to each of multiple similar objects moving continuously and rapidly in a series manner such as on a production line. For example, continuous ink-jet printing is frequently used for on-line application of production code to canned goods and medical products.
Continuous ink-jet printing requires an uninterrupted delivery of a continuous stream of liquid ink from a reservoir to a printer head. The printer head is typically stationary. The printer head controllably disintegrates the stream into a continuous series of discrete microdroplets of liquid ink. The trajectory of each microdroplet is instantaneously adjusted. Certain microdroplets are directed to deposit on preselected locations on each object being printed so as to form the desired printed pattern on the surface of the object. Alphanumeric print (and many other printable patterns such as bar code) are discontinuous; also, printing the same pattern on a series of objects moving past the printer head inherently requires temporary interruptions in the flow of ink from the printer head to the objects being printed. Hence, any microdroplets not destined to form part of the printed pattern on the surface of the object must be scavenged while in flight. Scavenging is usually effected by directing unused microdroplets to a "gutter." Ink collected in the gutter is returned, usually by pumping, to the ink reservoir used to supply ink to the printer head.
Ink collected in the gutter usually contains a substantial quantity of air bubbles. The presence of bubbles places unusual demands upon the type and features of the pump employed for returning the ink to the reservoir. In contrast, pumping ink from the reservoir to the printer head usually does not present a problem.
In certain conventional continuous ink-jet printing systems, gear pumps are used for both pumping tasks. Alternatively, in other conventional systems for continuous ink-jet printing, a gear pump is employed for delivering ink from the reservoir to the printing head and a venturi, actuated by a stream delivered by the gear pump, is used to withdraw collected ink from the gutter. In such a system, proper operation of the venturi requires a pumping capacity, substantially greater than what is required to provide ink to the printing head, to create a sufficiently reduced pressure in the venturi.
Ongoing efforts to increase the efficiency and lower costs of equipment such as medical equipment and continuous ink-jet printing systems has stimulated interest in various hydraulic, including pump, improvements. For example, manufacturers have tried using only one pump motor coupled to two separate pump heads, thereby eliminating the cost of a separate pump motor for each pump head. Whereas efforts to date have been beneficial, further improvements are desired.